Fixed `versionadded` inconsistencies in Symfony 2.5+

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.5+
| Fixed tickets | N/A

Same as symfony/symfony-docs#3471 - Founded some `versionadded` blocks that does not follow the "standard" of having the ".. was introduced in Symfony 2.x" or similar sentence.